linux-kernel - Re: [PATCH v2 2/2] mm: memcg: introduce new event to trace shrink

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Message-ID: <20231122185727.vcfg56d7sekdfhnm@CAB-WSD-L081021>
Date:   Wed, 22 Nov 2023 21:57:27 +0300
From:   Dmitry Rokosov <ddrokosov@...utedevices.com>
To:     Michal Hocko <mhocko@...e.com>
CC:     <rostedt@...dmis.org>, <mhiramat@...nel.org>, <hannes@...xchg.org>,
        <roman.gushchin@...ux.dev>, <shakeelb@...gle.com>,
        <muchun.song@...ux.dev>, <akpm@...ux-foundation.org>,
        <kernel@...rdevices.ru>, <rockosov@...il.com>,
        <cgroups@...r.kernel.org>, <linux-mm@...ck.org>,
        <linux-kernel@...r.kernel.org>, <bpf@...r.kernel.org>
Subject: Re: [PATCH v2 2/2] mm: memcg: introduce new event to trace
 shrink_memcg

On Wed, Nov 22, 2023 at 02:24:59PM +0100, Michal Hocko wrote:
> On Wed 22-11-23 13:58:36, Dmitry Rokosov wrote:
> > Hello Michal,
> > 
> > Thank you for the quick review!
> > 
> > On Wed, Nov 22, 2023 at 11:23:24AM +0100, Michal Hocko wrote:
> > > On Wed 22-11-23 13:01:56, Dmitry Rokosov wrote:
> > > > The shrink_memcg flow plays a crucial role in memcg reclamation.
> > > > Currently, it is not possible to trace this point from non-direct
> > > > reclaim paths.
> > > 
> > > Is this really true? AFAICS we have
> > > mm_vmscan_lru_isolate
> > > mm_vmscan_lru_shrink_active
> > > mm_vmscan_lru_shrink_inactive
> > > 
> > > which are in the vry core of the memory reclaim. Sure post processing
> > > those is some work.
> > 
> > Sure, you are absolutely right. In the usual scenario, the memcg
> > shrinker utilizes two sub-shrinkers: slab and LRU. We can enable the
> > tracepoints you mentioned and analyze them. However, there is one
> > potential issue. Enabling these tracepoints will trigger the reclaim
> > events show for all pages. Although we can filter them per pid, we
> > cannot filter them per cgroup. Nevertheless, there are times when it
> > would be extremely beneficial to comprehend the effectiveness of the
> > reclaim process within the relevant cgroup. For this reason, I am adding
> > the cgroup name to the memcg tracepoints and implementing a cumulative
> > tracepoint for memcg shrink (LRU + slab)."
> 
> I can see how printing memcg in mm_vmscan_memcg_reclaim_begin makes it
> easier to postprocess per memcg reclaim. But you could do that just by
> adding that to mm_vmscan_memcg_reclaim_{begin, end}, no? Why exactly
> does this matter for kswapd and other global reclaim contexts? 

>From my point of view, kswapd and other non-direct reclaim paths are
important for memcg analysis because they also influence the memcg
reclaim statistics.

The tracepoint mm_vmscan_memcg_reclaim_{begin, end} is called from the
direct memcg reclaim flow, such as:
    - a direct write to the 'reclaim' node
    - changing 'max' and 'high' thresholds
    - raising the 'force_empty' mechanism
    - the charge path
    - etc.

However, it doesn't cover global reclaim contexts, so it doesn't provide
us with the full memcg reclaim statistics.

-- 
Thank you,
Dmitry